1. Field of the Invention
The invention generally concerns a process for recognition of reference marks according to the pre-characterizing portion of patent claim 1, as well as a process for measuring or surveying objects.
2. Description of the Related Art Reference marks and processes of this type are already generally known from Ahn, S. J. (1997)-“Kreisförmige Zielmarke” (Circular-Shaped Measuring Marks); 4.ABW-Workshop, T A Esslingen, 22-23 Jan. 1997.
Reference marks and processes of this type serve for example for reducing the time sequence for measuring or surveying tasks in the industry. Lately it has primarily been optical measuring processes such as photogrammetry and strip or line projection that have received increasing importance. Basic requirements for this type of process include a robust design of the measuring process and a substantial automation. There is often a requirement for a rapid, comprehensive 3D measurement of larger objects, which must subsequently be available as a CAD data record. Optical measuring systems based on the principle of line-projection provide comprehensive 3D data covering the surface of the object being measured. The measurement volume of this system, which as a rule is limited, requires important supplemental information, such as connection points of an overall coordinate system, which makes possible a fitting assembly of multiple sets of measurements. The information necessary therefore, such as the 3D coordinates of pass-points, are provided by photogrammetry. With reference to the ability to automate the photogrammetric measuring and evaluation process, the measurement or gauging of the image receives a particular significance. With the increasing number of the image points to be measured or, as the case may be, the number of pictures of a total set, the investiture for identification of homologous image points increases dramatically. In order to keep the time requirement therefore as small as possible, reference marks are increasingly employed in practice which exhibit, besides a usually circular-shaped survey or index mark to be detected, also information-containing encoded marks. These encoded marks which can take various forms are designed to serve either for the improved visual oversight in the course of an image measurement carried out interactively by the user (“clicking” on homologous image points) or for an automated recognition of reference marks. In practice a number of problems occur in the recognition of reference marks due to the influence of geometric disturbances, such as for example rotation and/or distortion of the reference mark. In order to make it possible for a computer to compensate for these types of disturbances, it is advantageous when orientation features are supplementally provided on the reference marks.
The flexibility and robustness of an automated reference mark recognition system is determined in particular by the characteristics of the encoding employed. The value range (code depth) of the ring coding system employed widely in practice is limited, since only a limited number of encoding segments can be resolved. An increase of the data or value range is relatively inconvenient and would necessarily require a substantial. enlargement of the reference mark. Another factor influencing the automatic reference mark recognition system is the quality of the encoded image. Important influence factors thereon are geometric distortions, for example brought about by rotation, changes in scale, shearing, compression or perspective distortion of the referenced mark, or radiometric disturbances such as image static, reflections, lack of contrast, local contrast differences, and image sharpness or even partial covering over of the referenced mark. A compensation for this type of disturbance factor would substantially improve the automated reference mark recognition.
Erroneous recognition of a reference mark has a substantially larger error-influence on the subsequent evaluation process, for example on the surveying of an object, than would a rejected reference mark which is thus not taken into consideration (Goding, R. (1997) “Neue Aufnahme-und Auswertetechniken in der RolleiMetric Close Range Workstation (CDW)” (New Recording and Evaluation Techniques in the RolleiMetric Close Range Workstation) firm brochure of the company Rollei Photo Technique). This type of erroneous reading can be greatly reduced by determining a recognition quality, which makes it possible for the user to set thresholds for the rejection of the reference mark.